The sentence "He _____ (write) to me every month." requires determining the correct tense in the context of habitual action. In English grammar, when expressing habitual actions or routines, the simple present tense is typically used. Let's analyze the available options to find the suitable one:
- is writing: Present continuous tense, used for actions happening right now or around the current period, not suitable for describing habitual actions.
- writes: Simple present tense, used for habitual actions, routines, and general truths. This is the most appropriate choice for the given sentence.
- has been writing: Present perfect continuous tense, used for actions that started in the past and are still continuing or have recently finished, which doesn't fit the habitual context.
Therefore, the correct option to fill in the blank is: writes. The complete sentence is: "He writes to me every month."